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Properties of Steel
The factors influencing the properties of steel are chemical composition, heat treatment, and mechanical work.
The presence of carbon in steel gives high degree of hardness and strength. The addition of carbon to iron decreases the malleability and ductility of the metal, and reduces its permeability to magnetic forces.
The tensile strength of hot rolled steel bars is maximum between 1.0 and 1.2 per cent carbon. The elastic limit and the ultimate strength of steel increase with carbon content but at a lower rate. The compressive strength of steel increases directly with carbon content up to 1.0 per cent. The shear strength of steel also increases with the carbon content. The ratio of shear strength to the tensile strength is 0.80 for medium and low carbon steels and 0.60 for high carbon steels. The modulus of elasticity is nearly same for tension and compression and is practically independent of the carbon content.
The ductility of steel decreases markedly as the carbon content increases. The resistance of steel to heavy shocks or blows decreases with increase of carbon content.
Effects of Principal Impurities on Steel: It is not feasible to entirely remove impurities in making either iron or steel. The final product always contains small percentages of the metallic impurities like silicon, manganese, sulphur, and phosphorus besides iron and carbon. Occasionally small percentages of copper and arsenic are also present. In well made steel these impurities generally range between 0.2 and 1.0 per cent and their resultant effect on the constitution of steel is often small. Of the common impurities, Phosphorus cannot be eliminated in the process of manufacture, whereas most of the silicon and manganese are introduced to improve the metal.
Silicon is often added to molten metal to remove oxygen and diminish blow holes. In structural steel it rarely exceeds 0.25 per cent. Silicon up to 1.75 per cent appears to increase both ultimate strength and elastic limit without decreasing ductility.
Phosphorus is considered to promote enlargement of the grains and thus produce brittleness. The ductility of low-carbon steel decreases slightly by the presence of 0.3-0.5 per cent phosphorus. However, yield point, ultimate strength and hardness of steel are increased. Resistance to shock is also reduced by 0.1 per cent phosphorus and the metal is rendered cold short (i.e., brittle when cold). A decrease in toughness appears to be more pronounced in high-carbon than in low-carbon steels. The maximum limits for phosphorus are: for inferior grades of structural steel 0. 1, for best grades of structural steel 0.055, and 0.02 per cent for tool steels.
Sulphur readily combines with iron to form iron sulphide (FeS) which, when present in iron or steel, has a tendency to segregate and form brittle networks at the grain boundaries. On account of its low melting point, iron sulphide causes lack of cohesion between adjacent grains when heated above a red heat. Such brittleness at high temperature is termed as red shortness which makes steel or iron hard to roll or forge. Manganese sulphide has a much higher melting point than iron sulphidle and does not render ferrous metals red short. Therefore, inasmuch as manganese has a very powerful affinity for sulphur, it is possible to relieve red shortness by adding sufficient quantity of manganese to the molten metal to combine with sulphur. Theoretically the ratio of manganese to sulphur should be 1.70 to 1.0 in order to form manganese sulphide and completely satisfy sulphur. Less than 0.15 per cent sulphur content hardly exercises any appreciable effect on the mechanical properties of steel. When sulphur is present along with manganese it improves the machineability of steel.
Manganese has strong affinity for oxygen and sulphur and acts as a cleanser of the molten metal by withdrawing much of the undesirable impurities into the slag. Manganese increases the tensile strength, hardenability and dilutes the effect of sulphur. When more manganese is present than required for sulphur and oxygen the excess manganese forms carbide and acts as hardener. Copper increases resistance to corrosion when present in small percentage. Arsenic has a tendency to raise the strength and brittleness. Non-metallic Impurities are mechanically suspended in the metal and are often called slag inclusions causing brittleness.
The object of heat treatment is to develop desired properties in steel. The properties of steel can be controlled and changed as well by various heat treatments. A steel of given composition may be made soft, ductile and tough by one heat treatment, and the same steel may be made relatively hard and strong by another. Heat treatment affects the nature, amount, and character of the metallographic properties.
Heat treatment influences the solubility relations of the constituents, changes the crystallization either with respect to form or degree of aggregation and introduces or relieves internal stresses in the metal. The heat treatment process consists in subjecting, a metal to definite temperature-time course.
Some of the principle purposes of heat treatment are as follows.
1. To enhance properties such as strength, ductility, hardness and toughness.
2. To relieve internal stresses and strains.
3. To refine the grain.
4. To remove gases.
5. To normalize steel after heat treatment.
This heat treatment consists of heating the steel above the upper critical temperature holding at that temperature until phase equilibrium has been established, and then quenching rapidly to produce a martensite structure. Martensite is the chief constituent of hardened steel and is fibrous or needle like structure. Hardened steel is very brittle and cannot be used for practical purposes. The quenching medium is usually brine, water or oil, depending on the desired cooling rate.
The objective of this treatment may be to secure a given hardness to a desired depth in steel. But in most instances the hardening treatment may simply be considered as starting point from which better combinations of desired properties may be secured by subsequent heat treatment. Fully hardened steel are not suitable for most commercial uses because they are hard and brittle and have poor toughness.
A plain carbon steel that has been hardened is in metastable condition or equilibrium. If this hardened steel is reheated to some temperature below the critical range, a more stable condition will be obtained. Since hardened steels do not usually have the combination of properties desired for specific uses, modification is affected by tempering.
When a thick piece of steel is cooled rapidly it develops additional strains as the surface cools quicker than the interior. To relieve this strain, steel is subjected to the process tempering which consists in slowly heating the steel to a predetermined subcritical temperature and then cooling it slowly. This temperature varies from 100 o C to 700 o C. The higher the temperature of tempering the softer is the product. The properties like toughness and ductility are automatically introduced with release of strain.
It is a general term used for heating and slow cooling of metal, glass or any other material, which has developed strain due to rapid cooling.
The process consists of heating the steel to a temperature below the critical range, but high enough to obtain strain re-crystallization and then cooled in any manner. The exact heating temperature depends on the composition of steel and the amount of work that it has received, but is frequently between 500 o C to 600 o C. Annealing of steel in addition to removing strain introduces one or more of the following properties.
1. Introduces softness, ductility, and malleability.
2. Alters electrical, magnetic, and other physical properties.
3. Produces a definite microstructure and grain refinement.
4. Removes gases.
Full annealing consists of heating iron alloy 20 o C to 50 o C above critical temperature range, holding at that temperature for the required period of time to convert it to austenite followed by slow cooling. Full annealing usually decreases hardness, strength, and resistance to abrasion, and increases ductility and merchantability.
It consists in heating steel above critical range and cooling rapidly in air, but at rate slower than the critical cooling rate. The purpose of this heat treatment is to refine the grain structure resulting from rolling, forging or other manufacturing processes.
Steel products are made by casting molten refined steel of suitable composition into the desired form or by mechanically working steel form the ignot through many intermediate forms to the desired product. Mechanical work may be hot or cold. Mechanical working involves many stages of hot working and may or may not include eventual cold working.
The most important methods of hot working steel are hot rolling, hammer forging, hydraulic and mechanical press forging, and hot extrusion. Miscellaneous hot working methods include hot spinning, hot deep drawing, hot flanging and hot bending, Heat treatment after hot working is seldom used with low-carbon steels, whereas high-carbon steels are always hardened and tempered.
The principle methods of cold working steel are cold rolling, cold drawing and cold extrusion. The cold working methods are used to provide increased strength, accurate dimensions, and bright and scale free surfaces. Thin sheets and small diameter wires are produced by cold-working methods. Cold working results in increased density, hardness, and brittleness, and produces an internally strained condition in the steel.
Mechanical work alters the form of the crystalline aggregate and introduces internal stresses. Cold rolling increases the tensile elastic limit from 15 to 97 per cent and tensile strength from 20 to 45 per cent. In elastic resilience the cold-rolled metal is superior to the hot-rolled, whereas in energy of rupture it is inferior to the hot-rolled metal. The modulus of elasticity is slightly increased by cold rolling. Practically, metals are rolled, forged, drawn, stamped and pressed.
Most of steel building components-beams, rails, steels, bars, reinforcement, pipes- are manufactured by rolling. Rivets and bolts are made by forging operations. Thin-walled items (tubes) and round, square, hexagonal rods of small cross-sectional areas (up to 10 mm2) are manufactured by drawing. Stamping and pressing increases the buckling strength of plates to be used for making them suitable for steel tanks and containers. Steel trusses, towers, tanks, bridges and frames of multistorey buildings are some of the examples of structures made of steel. The most common and important application of steel in buildings is the rolled steel sections and reinforcing bars and are described in the following sections.
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